Conventionally, bicycle derailleurs constructed as described above, which, when the linkage mechanism is deformed, change the moving direction of the chain guide from the axial direction of the multistage sprocket assembly to the radial direction thereof in the plane perpendicular to the axis of the sprocket assembly, or to a slantwise direction of the axis of the sprocket assembly, has been disclosed in Japanese Patent Publication Gazette No. Sho 55-22,315 and is well known.
This type of conventional derailleur, a shown in FIGS. 11 and 12, is provided with two first and second connecting members 105 and 106 which are not part of a linkage mechanism 100 which comprises four members: a base member 101, linkage members 102 and 103 and a movable member 104, with linkage members 102 and 103 being pivoted at both ends to the connecting members 105 and 106 respectively. The first connecting member 105 is rotatably mounted to the base member 101 and the second connecting member 106 is similarly mounted to the movable member 104 through adjusting shafts 111 and 112 extending perpendicular to pivot shafts 107 through 110 for the linkage members 102 and 103 and lengthwise thereof. Also, an angular orientation of the first connecting member 105 with respect to the base member 101 and that of the second connecting member 106 with respect to the movable member 104 are made adjustable respectively. Accordingly, the first connecting member 105 can be adjusted in its angle with respect to the base member 101, thereby enabling a slant angle of the respective pivot shafts 107 to 110 at the linkage members 102 and 103 and the chain guide 113 supported to the movable member 104 to be changed in the moving direction with respect to the multistage sprocket assembly. In addition, during the adjustment, it is required that the second connecting member 106 be rotated an amount equal to rotation of the first connecting member 105 so as to adjust the axis of each pulley of chain guide 113 to be parallel to the axis of the multistage sprocket assembly. Thus, even when the multistage sprocket assembly is changed in construction to a wide-type sprocket assembly (a difference in tooth number of each sprocket is large) or a close-type sprocket assembly (the difference in tooth number is small), an interval between a guide pulley 51 and each sprocket can be made about constant.
In such conventional derailleur, however, when the connecting members 105 and 106 are rotated to change the moving direction of the chain guide 113, a slant angle of each pivot shaft at the linkage members 102 and 103 changes with the result that an exact amount of axial movement of the chain guide with respect to a swinging angle of the respective linkage members 102 and 103 changes. Therefore, an amount of axial movement of the chain guide 113 for the multistage sprocket assembly changes with respect to the operational stroke of a control wire to deform the linkage mechanism, thereby creating a problem in that the operation stroke of the control wire must be changed.
This problem can be solved, for example, by making the linkage members 102 and 103 adjustable in length so as to change the length thereof, or by changing the lever stroke of a control lever to operate the control wire. In the former case, the derailleur not only is large in size as a whole but also has a poor appearance. In the latter case, an adjustable control lever must be used so that a control lever provided with a positioning mechanism cannot be used, and also since swinging angles of the linkage members 102 and 103 increase during the shifting of a chain to a larger diameter sprocket, a torsion angle of a return spring increases for the derailleur provided therewith, thereby creating a problem in that the return spring has diminished durability. Also, for a control lever provided with an indicator for indicating the operating position of the control lever, the indicator may be shifted, thereby creating a problem in that the control lever provided with the positioning mechanism cannot be used.
The conventional derailleur, which is provided with the two connecting members 105 and 106 separate from the linkage mechanism 100, is both complicated in construction and also requires troublesome adjustment of the axis of each pulley of the chain guide when they are changed in their moving directions. Also, there is a risk that the connecting members 105 and 106, during use, will carelessly rotate to change the moving direction and parallel relationships of the chain guide 113.